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18.2: Structure and Function of Blood Vessels

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    22383
  • By the end of this section, you will be able to:

    • Compare and contrast the three tunics that make up the walls of most blood vessels
    • Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function
    • Compare and contrast the three types of capillaries on the basis of structure, location, and function
    • Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains
    • Compare and contrast veins, venules, and venous sinuses on the basis of structure, location, and function
    • Discuss several factors affecting blood flow in the venous system

    Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules, small blood vessels that carry blood to a vein, a larger blood vessel that returns blood to the heart.

    Blood flow is the movement of blood through a vessel, tissue, or organ. The slowing or blocking of blood flow is called resistance. Blood pressure is the force that blood exerts upon the walls of the blood vessels or chambers of the heart. The components of blood pressure include systolic pressure, which results from ventricular contraction, and diastolic pressure, which results from ventricular relaxation. Pulse, the expansion and recoiling of an artery, reflects the heartbeat. In the arterial system, vasodilation and vasoconstriction of the arterioles is a significant factor in systemic blood pressure: slight vasodilation greatly decreases resistance and increases flow, whereas slight vasoconstriction greatly increases resistance and decreases flow. In the arterial system, as resistance increases, blood pressure increases and flow decreases. In the venous system, constriction increases blood pressure as it does in arteries; the increasing pressure helps to return blood to the heart. In addition, constriction causes the vessel lumen to become more rounded, decreasing resistance and increasing blood flow.

    Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit (Figure \(\PageIndex{1}\)). The systemic circuit begins in the left atrium of the heart and ends in the venae cavae and coronary sinus that drain into the right atrium. Systemic arteries provide blood rich in oxygen to the body’s tissues; this blood is often referred to as oxygenated blood. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells; this blood is often referred to as deoxygenated blood. In contrast, the pulmonary circuit begins in the right atrium and ends in the pulmonary veins that drain into the left atrium. Pulmonary arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features.

    Pulmonary and systemic circuits of blood flow with artery, capillary bed, and vein for each. Systemic circuit includes four examples supplying upper body, kidney, lower body, and the two capillary beds of the hepatic portal system.Figure \(\PageIndex{1}\): Cardiovascular Circulation in Pulmonary and Systemic Circuits. The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. Blood flows in one direction to and from capillary beds, each with a network of interconnected capillaries, that are positioned between the arterial and venous sides of circulation throughout the body to allow for gas and nutrient exchange. In the pulmonary circuit, deoxygenated blood flows on the arterial side of the circuit while oxygenated blood flows on the venous side. In the systemic circuit, oxygenated blood flows on the arterial side of the circuit while deoxygenated blood flows on the venous side. In the figure, the arrows indicate the direction of blood flow and the colors show the relative levels of oxygen concentration. (Image credit: "Cardiovascular Circulation in Pulmonary and Systemic Circuits" by Julie Jenks / A derivative from the original work)

    Structural Characteristics of Vessels

    Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure \(\PageIndex{2}\)). Each type of vessel has a lumen—a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.

    Structure and proportions of the three vessel tunics and lumen of arteries and veins are compared via diagram and micrograph.Figure \(\PageIndex{2}\): Structure of Blood Vessels. (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them. (c) A micrograph shows a similarly sized artery and vein. Note the relative differences in wall thickness and compare the round lumen of the artery to the flattened lumen of the vein. LM × 160. (Image credit: "Comparison of Artery and Vein" by Julie Jenks / A derivative from the original work / Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

    By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.

    The walls of arteries and veins are largely composed of living cells and their products (including collagen and elastic fibers); the cells require nourishment and produce waste. Since blood passes through the larger vessels relatively quickly, there is limited opportunity for blood in the lumen of the vessel to provide nourishment to or remove waste from the vessel’s cells. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Larger arteries and veins contain small blood vessels within their walls known as the vasa vasorum—literally “vessels of the vessel”—to provide them with this critical exchange. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel (see Figure \(\PageIndex{2}\)) or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since their location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum.

    Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica, for the garments first worn by ancient Romans). From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa (see Figure \(\PageIndex{2}\)). Table \(\PageIndex{1}\) compares and contrasts the tunics of the arteries and veins.

    Table \(\PageIndex{1}\): Comparison of Tunics in Arteries and Veins
    Tunic (Layer) Arteries Veins
    General Appearance Thick walls with small lumens
    Generally appear rounded
    Thin walls with large lumens
    Generally appear flattened
    Tunica Intima Endothelium usually appears wavy due to constriction of smooth muscle
    Internal elastic membrane present in larger vessels
    Endothelium appears smooth
    Internal elastic membrane absent
    Tunica Media

    Normally the thickest layer in arteries
    Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart)
    Nervi vasorum and vasa vasorum present
    External elastic membrane present in larger vessels

    Normally thinner than the tunica externa
    Smooth muscle cells with elastic and collagen fibers
    Nervi vasorum and vasa vasorum present
    External elastic membrane absent
    Tunica Externa Normally thinner than the tunica media in all but the largest arteries
    Collagen and elastic fibers
    Nervi vasorum and vasa vasorum present
    Normally the thickest layer in veins
    Collagen and smooth fibers predominate
    Some smooth muscle fibers
    Nervi vasorum and vasa vasorum present

    Tunica Intima

    The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagen fibers beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease.

    Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagen fibers to provide additional strength.

    In larger arteries, there is also a thick, distinct layer of elastic connective tissue known as the internal elastic membrane (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain one-way valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.

    Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.

    Tunica Media

    The tunica media is the substantial middle layer of the vessel wall (see Figure \(\PageIndex{2}\)). It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal smooth muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. Both vasoconstriction and vasodilation are regulated in part by small vascular nerves, known as nervi vasorum, or “nerves of the vessel,” that run within the walls of blood vessels. Neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration.

    The smooth muscle layers of the tunica media are supported by a framework of collagen fibers that also binds the tunica media to the inner and outer tunics. Along with the collagen fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane (also called the external elastic lamina), which also appears wavy in slides. This structure is not usually seen in smaller arteries, nor is it seen in veins.

    Tunica Externa

    The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of dense irregular connective tissue composed primarily of collagen fibers. Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position. If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.

    Arteries

    An artery is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery (Figure \(\PageIndex{3}\)). Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump and maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen gives it a low resistance and enables it to accept a large volume of blood from the heart which is conducted to smaller branches within regions of the body.

    Comparison of vessel tunics in different types of arteriesFigure \(\PageIndex{3}\): Types of Arteries and Arterioles. A comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. The scale of each vessel's wall has been adjusted for comparison in this illustration. The tunica media is thickest in muscular arteries in proportion to the diameter of the lumen. Tunica externa is very thin and incomplete in arterioles. (Image credit: "Types of Arteries and Arterioles" by Julie Jenks / A derivative from the original work)

    Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction which controls blood flow to individual organs. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important.

    Notice that although the distinctions between elastic and muscular arteries are important, there is no “line of demarcation” where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles that deliver blood to capillaries within specific organs and tissues. For this reason, a muscular artery is also known as a distributing artery.

    Arterioles

    An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin as arterioles are supported and held in place by their positioning within organs and tissues (see Figure \(\PageIndex{3}\)).

    With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow to capillary beds as well as regulation of systemic blood pressure.

    DISORDERS OF THE...

    Cardiovascular System: Arteriosclerosis

    Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. This helps promote blood flow. In arteriosclerosis, compliance is reduced, and pressure and resistance within the vessel increase. This is a leading cause of hypertension and coronary heart disease, as it causes the heart to work harder to generate a pressure great enough to overcome the resistance.

    Arteriosclerosis begins with injury to the endothelium of an artery, which may be caused by irritation from high blood glucose, infection, tobacco use, excessive blood lipids, and other factors. Artery walls that are constantly stressed by blood flowing at high pressure are also more likely to be injured—which means that hypertension can promote arteriosclerosis, as well as result from it.

    Recall that tissue injury causes inflammation. As inflammation spreads into the artery wall, it weakens and scars it, leaving it stiff (sclerotic). As a result, compliance is reduced. Moreover, circulating triglycerides and cholesterol can seep between the damaged lining cells and become trapped within the artery wall, where they are frequently joined by leukocytes, calcium, and cellular debris. Eventually, this buildup, called plaque, can narrow arteries enough to impair blood flow. The term for this condition, atherosclerosis (athero- = “porridge”) describes the mealy deposits (Figure \(\PageIndex{4}\)).

    Atherosclerosis involves calcified, fatty plaques that build up in a damaged artery wall and narrow the diameter of the artery, causing ischemia.
    Figure \(\PageIndex{4}\): Atherosclerosis. (a) Atherosclerosis can result from plaques formed by the buildup of fatty, calcified deposits in an artery. (b) Plaques can also take other forms, as shown in this micrograph of a coronary artery that has a buildup of connective tissue within the artery wall. LM × 40. (Image credit: "Atherosclerosis" by OpenStax is licensed under CC BY 3.0; Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Sometimes a plaque can rupture, causing microscopic tears in the artery wall that allow blood to leak into the tissue on the other side. When this happens, platelets rush to the site to clot the blood. This clot can further obstruct the artery and—if it occurs in a coronary or cerebral artery—cause a sudden heart attack or stroke. Alternatively, plaque can break off and travel through the bloodstream as an embolus until it blocks a more distant, smaller artery.

    Even without total blockage, vessel narrowing leads to ischemia—reduced blood flow—to the tissue region “downstream” of the narrowed vessel. Ischemia in turn leads to hypoxia—decreased supply of oxygen to the tissues. Hypoxia involving cardiac muscle or brain tissue can lead to cell death and severe impairment of brain or heart function.

    A major risk factor for both arteriosclerosis and atherosclerosis is advanced age, as the conditions tend to progress over time. Arteriosclerosis is normally defined as the more generalized loss of compliance, “hardening of the arteries,” whereas atherosclerosis is a more specific term for the build-up of plaque in the walls of the vessel and is a specific type of arteriosclerosis. There is also a distinct genetic component, and pre-existing hypertension and/or diabetes also greatly increase the risk. However, obesity, poor nutrition, lack of physical activity, and tobacco use all are major risk factors.

    Treatment includes lifestyle changes, such as weight loss, smoking cessation, regular exercise, and adoption of a diet low in sodium and saturated fats. Medications to reduce cholesterol and blood pressure may be prescribed. For blocked coronary arteries, surgery is warranted. In angioplasty, a catheter is inserted into the vessel at the point of narrowing, and a second catheter with a balloon-like tip is inflated to widen the opening. To prevent subsequent collapse of the vessel, a small mesh tube called a stent is often inserted. In an endarterectomy, plaque is surgically removed from the walls of a vessel. This operation is typically performed on the carotid arteries of the neck, which are a prime source of oxygenated blood for the brain. In a coronary bypass procedure, a non-vital superficial vessel from another part of the body (often the great saphenous vein) or a synthetic vessel is inserted to create a path around the blocked area of a coronary artery.

    Capillaries

    A capillary is a microscopic channel that supplies blood to the tissues, through a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation.

    The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. Some variation in wall structure is seen depending on the size of the capillary. In a large capillary, several endothelial cells bordering each other may line the lumen, while in a small capillary, there may be only a single cell layer that wraps around to contact itself.

    For capillaries to function, their walls must be leaky, or permeable, allowing some substances to pass through. There are three major types of capillaries, which differ according to their degree of permeability: continuous, fenestrated, and sinusoid capillaries (Figure \(\PageIndex{5}\)).

    Continuous Capillaries

    The most common type of capillary, the continuous capillary, is found in almost all vascularized tissues. Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes. Continuous capillaries not associated with the brain are rich in transport vesicles, contributing to either endocytosis or exocytosis. Those in the brain are part of the blood-brain barrier. Here, there are tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions called end feet; these structures combine to prevent the movement of nearly all substances.

    Comparison of anatomy of different types of capillaries
    Figure \(\PageIndex{5}\): Types of Capillaries. The three major types of capillaries: continuous, fenestrated, and sinusoid. (Image credit: "Three Major Capillary Types" by OpenStax is in the Public Domain)

    Fenestrated Capillaries

    A fenestrated capillary is one that has pores (or fenestrations) in addition to tight junctions in the endothelial lining. These make the capillary permeable to larger molecules. The number of fenestrations and their degree of permeability vary according to their location. Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood. They are also found in the choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands.

    Sinusoid Capillaries

    A sinusoid capillary (or sinusoid) is the least common type of capillary and the most permeable. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. This gives them an appearance similar to Swiss cheese. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells. Blood flow through sinusoids is very slow, allowing more time for exchange of gases, nutrients, and wastes. Sinusoids are found in the liver and spleen, bone marrow, lymph nodes (where they carry lymph, not blood), and many endocrine glands including the pituitary and adrenal glands. Without these specialized capillaries, these organs would not be able to provide their myriad functions. For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary; they cannot pass through the small openings of continuous or fenestrated capillaries. The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation.

    Metarterioles and Capillary Beds

    A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary. Slightly larger than the typical capillary, the smooth muscle of the tunica media of the metarteriole is not continuous but forms rings of smooth muscle (sphincters) prior to the entrance of the capillaries. Each metarteriole arises from a terminal arteriole and branches to supply blood to a capillary bed that may consist of 10–100 capillaries depending on the metabolic needs of the surrounding tissues.

    The precapillary sphincters, circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies. Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself. Normally, the precapillary sphincters are closed. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more (Figure \(\PageIndex{6}\)). If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. This creates what is known as a vascular shunt. In addition, some regions contain one or more arteriovenous anastomoses, which can be opened to bypass the capillary bed, skipping perfusion and leading directly to the venous system.

    Although you might expect blood flow through a capillary bed to be smooth, in reality, it moves with an irregular, pulsating flow. This pattern is called vasomotion and is regulated by chemical signals that are triggered in response to changes in internal conditions, such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels. For example, during strenuous exercise when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic acid levels all increase, the capillary beds in skeletal muscle are open, as they would be in the digestive system when nutrients are present in the digestive tract. During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes.

    Anatomy of a Capillary Bed
    Figure \(\PageIndex{6}\): Capillary Bed. In a capillary bed, arterioles give rise to metarterioles. Precapillary sphincters located at the junction of a metarteriole with a capillary regulate blood flow. A thoroughfare channel connects the metarteriole to a venule. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom. (Image credit: "Capillary Bed" by OpenStax is licensed under CC BY 3.0)

    Venules

    A venule is an extremely small vein, generally 8–100 micrometers in diameter. Postcapillary venules drain multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin or non-existent middle layer with a few smooth muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa (Figure \(\PageIndex{7}\)). Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze between adjacent cells to enter the tissue fluid.

    Veins

    A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (see Figure \(\PageIndex{2}\)). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity (Figure \(\PageIndex{7}\)).

    Comparison of Types of Veins
    Figure \(\PageIndex{7}\): Comparison of Veins and Venules. Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins. (Image credit: "Large Medium Vein Venule" by OpenStax is licensed under CC BY 3.0)

    Table \(\PageIndex{2}\) compares the features of arteries and veins.

    Table \(\PageIndex{2}\): Comparison of General Features of Arteries and Veins

    Arteries Veins
    Direction of blood flow Conducts blood away from the heart Conducts blood toward the heart
    General appearance Rounded Irregular, often collapsed
    Pressure High Low
    Wall thickness Thick Thin
    Relative oxygen concentration

    Higher in systemic arteries

    Lower in pulmonary arteries

    Lower in systemic veins

    Higher in pulmonary veins

    Valves Not present Present most commonly in limbs and in veins inferior to the heart

    Venous System

    The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic “pumps” increase pressure in the venous system. The use of the term “pump” implies a physical device that speeds flow. These physiological pumps are less obvious.

    Skeletal Muscle Pump

    In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump (Figure \(\PageIndex{8}\)), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness.

    Figure \(\PageIndex{8}\): Skeletal Muscle Pump. The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area. This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction. (Image credit: "Skeletal Muscle Vein Pump" by OpenStax is licensed under CC BY 3.0)

    Respiratory Pump

    The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity. The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax. The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins.

    DISORDERS OF THE...

    Cardiovascular System: Edema and Varicose Veins

    Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries. Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema.

    Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet (like most health professionals). However, clinical edema goes beyond normal swelling and requires medical treatment. Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated.

    Photograph of left leg with varicose veins visible as lumps under the skin.
    Figure \(\PageIndex{9}\): Varicose Veins. Varicose veins are commonly found in the superficial veins of the lower limbs. Defective valves cause localized pooling of blood in the veins that can lead to distention and deformity of the vessel appearing as lumps under the surface of the skin. (Image credit: "Left Calf and Varicose Veins" by Thomas Kriese is licensed under CC BY 2.0)

    Edema may be accompanied by varicose veins, especially in the superficial veins of the legs (Figure \(\PageIndex{9}\)). This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition. Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation. There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot.

    Veins as Blood Reservoirs

    In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time (Figure \(\PageIndex{10}\), Table \(\PageIndex{3}\)). Their ability to hold this much blood is due to their high capacitance, that is, their capacity to distend (expand) readily to store a high volume of blood, even at a low pressure. The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels.

    Chart to visualize the distribution of blood flow that is presented in table format elsewhere.
    Figure \(\PageIndex{10}\): Graphical View of Distribution of Blood Flow. Blood spends a higher proportion of time in systemic veins than in any other part of the cardiovascular system. This chart shows the hierarchical percentages of blood flow found within each circuit of blood flow (inner ring), within the arteries, capillaries, and veins of each circuit (middle ring), and within each specific type of systemic vessel (outer ring). The same data used to create this graphic are given in Table \(\PageIndex{3}\). (Image credit: "Distribution of Blood Flow" by Julie Jenks is licensed under CC BY 4.0)

    Table \(\PageIndex{3}\): Distribution of Blood Flow
    Circuit of Blood Flow Vessel Category Specific Vessel Type Percentage of Blood Flow
    Systemic Circulation, 84% Systemic Arteries, 13% Aorta 2%
    Elastic Arteries 4%
    Muscular Arteries 5%
    Arterioles 2%
    Systemic Capillaries 7%
    Systemic Veins, 64% Large Veins 18%
    Large venous network (liver, bone marrow, integument) 21%
    Venules and medium-sized veins 25%
    Pulmonary Circulation, 9% Pulmonary Arteries 3%
    Pulmonary Capillaries 2%
    Pulmonary Veins 4%
    Heart 7%

    Concept Review

    Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.

    Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. Blood flow through vessels can be dramatically increased or decreased by vasoconstriction and vasodilation. The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Capillaries have only a tunica intima layer.

    Review Questions

    Q. The endothelium is found in the ________.

    A. tunica intima

    B. tunica media

    C. tunica externa

    D. lumen

    Answer

    Answer: A

    Q. Nervi vasorum control ________.

    A. vasoconstriction

    B. vasodilation

    C. capillary permeability

    D. both vasoconstriction and vasodilation

    Answer

    Answer: D

    Q. Closer to the heart, arteries would be expected to have a higher percentage of ________.

    A. endothelium

    B. smooth muscle fibers

    C. elastic fibers

    D. collagen fibers

    Answer

    Answer: C

    Q. Which of the following best describes veins?

    A. thick walled, small lumens, low pressure, lack valves

    B. thin walled, large lumens, low pressure, have valves

    C. thin walled, small lumens, high pressure, have valves

    D. thick walled, large lumens, high pressure, lack valves

    Answer

    Answer: B

    Q. An especially permeable type of capillary found in the liver and certain other tissues is called a ________.

    A. capillary bed

    B. fenestrated capillary

    C. sinusoid capillary

    D. metarteriole

    Answer

    Answer: C

    Critical Thinking Questions

    Q. Arterioles are often referred to as resistance vessels. Why?

    Answer

    A: Arterioles receive blood from arteries, which are vessels with a much larger lumen. As their own lumen averages just 30 micrometers or less, arterioles are critical in slowing down—or resisting—blood flow. The arterioles can also constrict or dilate, which varies their resistance, to help distribute blood flow to the tissues.

    Q. A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this?

    Answer

    A: This is a venule.

    Q. An obese patient comes to the clinic complaining of swollen feet and ankles, fatigue, shortness of breath, and often feeling “spaced out.” She is a cashier in a grocery store, a job that requires her to stand all day. Outside of work, she engages in no physical activity. She confesses that, because of her weight, she finds even walking uncomfortable. Explain how the skeletal muscle pump might play a role in this patient’s signs and symptoms.

    Answer

    A. People who stand upright all day and are inactive overall have very little skeletal muscle activity in the legs. Pooling of blood in the legs and feet is common. Venous return to the heart is reduced, a condition that in turn reduces cardiac output and therefore oxygenation of tissues throughout the body. This could at least partially account for the patient’s fatigue and shortness of breath, as well as her “spaced out” feeling, which commonly reflects reduced oxygen to the brain.

    Glossary

    arteriole
    (also, resistance vessel) very small artery that leads to a capillary
    arteriovenous anastomosis
    short vessel connecting an arteriole directly to a venule and bypassing the capillary beds
    artery
    blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel
    blood flow
    movement of blood through a blood vessel or the heart
    blood pressure
    the pressure exerted by the blood on the walls of a blood vessel or the heart
    capacitance
    ability of a vein to distend and store blood
    capacitance vessels
    veins
    capillary
    smallest of blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid
    capillary bed
    network of 10–100 capillaries connecting arterioles to venules
    continuous capillary
    most common type of capillary, found in virtually all tissues except epithelia and cartilage; contains very small gaps in the endothelial lining that permit exchange
    elastic artery
    (also, conducting artery) artery with abundant elastic fibers located closer to the heart, which maintains the pressure gradient and conducts blood to smaller branches
    external elastic membrane
    membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries
    fenestrated capillary
    type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials
    internal elastic membrane
    membrane composed of elastic fibers that separates the tunica intima from the tunica media; seen in larger arteries
    lumen
    interior of a tubular structure such as a blood vessel or a portion of the alimentary canal through which blood, chyme, or other substances travel
    metarteriole
    short vessel arising from a terminal arteriole that branches to supply a capillary bed
    microcirculation
    blood flow through the capillaries
    muscular artery
    (also, distributing artery) artery with abundant smooth muscle in the tunica media that branches to distribute blood to the arteriole network
    nervi vasorum
    small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls
    perfusion
    distribution of blood into the capillaries so the tissues can be supplied
    permeable
    allowing some substances to pass through
    precapillary sphincters
    circular rings of smooth muscle that surround the entrance to a capillary and regulate blood flow into that capillary
    respiratory pump
    increase in the volume of the thorax during inhalation that decreases air pressure, enabling venous blood to flow into the thoracic region, then exhalation increases pressure, moving blood into the atria
    sinusoid capillary
    rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs
    skeletal muscle pump
    effect on increasing blood pressure within veins by compression of the vessel caused by the contraction of nearby skeletal muscle
    thoroughfare channel
    continuation of the metarteriole that enables blood to bypass a capillary bed and flow directly into a venule, creating a vascular shunt
    tunica externa
    (also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries)
    tunica intima
    (also, tunica interna) innermost lining or tunic of a vessel
    tunica media
    middle layer or tunic of a vessel (except capillaries)
    vasa vasorum
    small blood vessels located within the walls or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels
    vascular shunt
    continuation of the metarteriole and thoroughfare channel that allows blood to bypass the capillary beds to flow directly from the arterial to the venous circulation
    vasoconstriction
    constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter
    vasodilation
    relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter
    vasomotion
    irregular, pulsating flow of blood through capillaries and related structures
    vein
    blood vessel that conducts blood toward the heart
    venous reserve
    volume of blood contained within systemic veins in the integument, bone marrow, and liver that can be returned to the heart for circulation, if needed
    venule
    small vessel leading from the capillaries to veins

    Contributors and Attributions